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I accidentally typed
cd /tmp | tail
which is of course not what I meant. But to my surprise, I then did not change directory. Why is this? For comparison
cd /tmp > /tmp/foo
does result in a change of directory, so it's not the output redirection per sé. What's going on here?
Pipes involve subshells, portions of which are not run in the parent shell and thus cannot affect the working directory of that parent shell process. This is how ZSH behaves:
% cd /tmp
% cd /var/tmp | tail
% pwd
/tmp
% echo foo | cd /var/tmp
(pwd now: /var/tmp)
% cd /tmp
% echo foo | cd /var/tmp | echo bar
bar
% pwd
/tmp
%
Note how the directory only changed when the cd was the last command in the pipeline; this was run in the parent shell proper and thus was able to change the working directory of that process.
A useful use of this feature usually involves an explicit subshell and commands run therein:
dowork | ( cd elsewhere && domorework ) | andyetmore
also be sure to error check the cd call instead of assuming it worked, unless you like rsync output sprayed all over /, or other such hypothetical messes...
tail would need to be in a subshell, but I don't understand by the cd must be in a subshell, for the pipe to work.
– gerrit
Jun 22 '18 at 23:17
cd /tmp | pwdexhibits the same behavior --pwdshows an unchanged working directory. – DopeGhoti Jun 22 '18 at 16:23